The present invention is directed to an expansion dowel assembly with an axially elongated anchor bolt at least partially cylindrically shaped and shell-shaped expansion elements with means on the anchor bolt for applying a load adjacent its trailing end. Adjacent the leading end of the anchor bolt are two axially extending conically shaped expansion members or cones arranged to move into the expansion elements for widening them. The expansion member closer to the leading end of the anchor bolt has a larger cone angle than the other expansion member. The expansion elements are connected to one another by axially extending webs.
Expansion dowel assemblies with a pair of serially arranged expansion cones are disclosed in DE-OS No. 28 28 983. Such dowel assemblies achieve higher extraction values compared to conventional expansion dowels utilizing a single expansion cone. This difference can be traced to the fact where two expansion members or cones are used, the expansion pressure in the borehole is distributed over a larger surface. In the case of conically shaped expansion members arranged serially in the axially direction, it is assured that at least one of the two expansion elements is expanded in the borehole. In the known expansion dowel assemblies, however, it is a matter of chance as to which of the two expansion elements first comes into contact with the borehole surface.
In the anchoring of this type of dowel assembly there are two different phases. For the initiation of the expanding process, a cone with a slight cone angle of about eight degrees at a maximum is required. Such a slender cone results in large re-expansion travel and a limited expansion force. For an optimum introduction force into the receiving material for the dowel assembly, an expansion member or cone with a cone angle greater than ten degrees is appropriate.
There is another known expansion dowel assembly disclosed in U.S. Pat. No. 4,579,490 with two conically shaped expansion members arranged one behind the other in the axial direction and each with a different cone angle. In this known arrangement, the cone angle of the leading expansion member is larger than that of the adjacent trailing expansion member. The shell-shaped expansion elements are interconnected by a web. With the appropriate matching of the axial spacing between the expansion elements and the expansion members it is assured at the outset that the trailing expansion member expands the trailing expansion element and thus fixes it axially in the receiving material. After a certain amount of travel, the leading expansion element is also expanded by the leading expansion member. Due to the rigid connection of the expansion elements by the web, the expansion of the elements is mutually affected, whereby an inadequate anchoring is achieved.